Light novel where people have abilities and magic and can level up to defeat monsters invading the world

Question

From what I can remember the novel is like solo leveling in the sense it is a world where people have abilities and magic and monsters are invading the world. The main character is one of the strongest but finds being the strongest makes you the loneliest.

His ability or job was magic swordsman and he is only just able to defeat a demon lord but it's still not enough.

At the start of the story him and a former friend are sent back in time to before the monsters came both with their memories of the future intact but he does not want the former friend knowing. This means he is a normal person again.

Major plot points I can remember are: The former friend gets the magic swordsman class and the MC gets a stronger one.

The MC can transport himself into the body of the demon lord who he fought in the future and there are more demon lords fighting for supremacy and the one he finds himself in had been asleep for 100 years.

At one point the MC as demon lord buys a dragon egg from a auction that is supposed be faulty but has twins instead off just one. One is magic orientated and the other has high stats in strength which is sent to the human world so he can take care of it.

The MC obtains a bracelet named Jörmungandr in the shape of a snake from a treasury.

Answer 1

Perhaps this is Colossus Hunter (2017).

The synopsis looks like a good match:

He returned from the hopeless future!

In the midst of the clash between gods and devils, Hansung begins his journey.

Oh Hansung, the ultimate hero bearing the class Magic Swordsman and the only titled Hero left in the world destroys the evil cult: The Order of Alethea who worship Chronos, the God of Time. Whom his friend Minshik is a part of. After the cult’s destruction, Minshik who is in a state beyond repair spoke out with these words: “If our God reversed the flow of time, I wanted to be a greater hero than you…”

From destroying a Demon Dragon to the Demon King, and finally, to where Hansung is killing things that are even worse, the hero is stuck in an endless cycle of suffering until he dies. In a miraculous event, Minshik’s ring, a ring Hansung kept after his friend’s death started to glow in his dying breath and caused him to reincarnate into the past. Unfortunately, Hansung and Minshik are reincarnated into the time where Hansung’s parents died in a car crash and he’s forced to live with his somewhat abusive uncle, wherein two years’ time many monsters will start invading. Minshik starts out immediately to gain the class Magic Swordsman and doesn’t realize Hansung also reincarnated. In his concealed state Hansung formulates his plan to save the world by going after an even stronger class.

I just read it up to chapter 6. Seems like he temporarily soul transfers with the Demon Lord and meets the Demon Lord's vassal. Here is a quote from this chapter:

“You have been slumbering for nearly 100 years, my Lord. There was not a day where I did not long for your return, oh wondrous one.”

Found by multiple reincarnation korean novel.